• Replicates in a semiconservative manner
• Involves 2 steps
1. Separating the DNA strands
2. Building the complimentary
strands

• SEPARATING THE DNA STRANDS
– Hydrogen bonds btw the two strands of nucleotides are broken by DNA helicase

• SEPARATING DNA STRANDS
– Single stranded binding proteins (SSBs) bind the two strands of nucleotides to prevent them from reforming H bonds

• SEPARATING DNA STRANDS
– As DNA unwinds, tension dvlp’s behind the replication fork

• SEPARATING DNA STRANDS
– DNA gyrase reduces the tension by splicing and rejoining the two strands once they have moved over one another

• The site which DNA helicase unwinds is called the origin of replication
• Prokaryotic cells have 1 origin of replication, while eurkaryotic cells have multiple origins of replication

• A replication bubble will form as DNA helicase unwinds the DNA
• The term “replication fork” is used to describe the edges of the replication bubble where the two strands are still joined

• The replication bubble will continue to get larger and larger until the entire molecule of
DNA has been reproduced

• DNA can only replicate in one direction:
– 5’ to 3’ direction
• As a result, the new strand must start at the 3’ end of the original strand

• A problem arises from the fact that DNA strands run antiparallel
• One of the new strands will be formed as a continuous molecule
• The other strand will be formed as a discontinuous molecule

• BUILDING THE COMPLIMENTARY STRAND
– The formation of a new strand of DNA requires the action of 4 different enzymes
1. Primase
2. DNA Polymerase III
3. DNA Polymerase I
4. DNA Ligase

• Responsible for laying down an RNA primer that will be used by DNA Polymerase III to build new strand
• Works on discontinuous strand

• Responsible for adding the correct nucleotide to the dvlping strand of DNA
• On continuous strand, it is used to build the first complimentary strand of DNA because it moves from 5’-3’

• Responsible for joining together all the different fragments in the growing strand of
DNA

• Also called the leading strand
• As the molecule of DNA is unwound, primase attaches an RNA primer to the 3’ end of the original DNA molecule
• DNA Polymerase III will start to build new strand in 5’-3’ direction

• Also called the lagging strand
• As the molecule of DNA is unwound, primase attaches an RNA primer to an exposed 3’ end of the original 5’-3’ strand
• DNA Polymerase III will start to build the new strand in 5’-3’ direction

• The fragments made are called Okazaki fragments
• After strands have been formed, DNA
POLYMERASE I removes the RNA primer and replaces it with proper nucleotides
• DNA Ligase attaches all fragment pieces into a single strand of DNA

• The enzyme backtracks past the nucleotide on the end of the DNA strand that is incorrectly paired and removes it
• Then continues to add more DNA nucleotides

1. Read pages 217- 222
2. Explain why single stranded binding proteins
(SSBs) are necessary in DNA replication.
3. Explain how the enzyme gyrase works.
4. Answer questions 2, 4-7 page 223